I really dont see the point of running around people, especially against a caster

There is none. When people start doing that to me I just spam CC them just to piss them off. I wont even try and kill them. Just rotate every single CC ability I have. Also its usually rogues but they are cowards anyways.

Lets say you are against a nub destro lock, and he is casting Chaos Bolt. You have no way to interupt his cast with your abilities, and you can't LoS him in time. However, you note that this lock is nub keyboard turner. Thus, you can run behind him, and "interupt" his spell cast as players can NOT attack targets behind them, except for a few DoT classes.

So essentially it helps to reduce incoming damage if done right as it takes a larger amount of reaction time to turn around and resume attacking. Of course, most melee suck at this, and hurt their damage output just as much as the opponents.

Anyone ever notice how the sun seems to shine silverish now? Didn't it used to shine goldish? PM me if you've noticed this.

Probably just a habitual thing so as to feel like they're ready to move or react to what the enemy does asap. Sometimes in a raid environment I feel as if I never stop moving then I'm far less likely to get hit by things - not because of the obvious fact that I'm just never in places where abilities target the ground under me, but because it forces me to continuously pay attention and not tunnel - and of course as a hunter I can do it without any real loss. I also instinctually move away from other people (aside from situations where stacking is intended) when they get close to me out of not wanting to get hit by things that target them. Of course those are different than PvP, somewhat similar. Or they might just be doing it because they are spazzy.

It is about position mainly, but there are many reasons to constantly try to get behind a target.

1.) Some melee attacks require you to be behind your target.
2.) You cannot parry from behind (my DK has like 20% parry in pvp gear)
3.) You cannot dodge an attack from behind. (casters may not have parry but they do have some small amount of dodge)
4.) You cannot cast most spells at targets behind you.
5.) Being in motion makes it hard to predict where you are going or what you are doing.
6.) Some attacks are ground targeted.
7.) Some melee have short melee range and it is easy to stop lots of incoming damage by being just a few yards away. (think WW monks).

So there are lots of reasons to constantly try to jockey for position behind your target, but there are people that don't understand all these reasons and either do it because they are hyperactive or even they just mimic what they see other pvpers do.

You'd be a fool to stand still. Movement is pretty huge, because positioning is. Versus bad casters it is very nice to make them flub a non-CC cast with a run through, and versus good ones I force them to play harder to land something. I'll eat up every thoughtcycle they have, and if it frustrates them, that's much better for me in all possible ways.

I think it comes from a combination of class rotation downtime and dealing with LOS for many spells. A lot of casters have to be facing their target when the cast goes off to hit them, so if ya keep jumping around it's SLIGHTLY harder for a crappy keyboard turner to defeat you. But let's be real, if you're good enough to jump around while fighting... AND facing a keyboard turner, do you REALLY need the advantage?

I also think it just comes from class downtime. I LOVE pressing buttons, even if it doesn't help, I just need to always be doing something when in combat. I think a lot of people are the same, so they jump around and mash spacebar during the rotation downtime so they're at least doing something!